Pyroelectric radiation receivers, i.e., radiation detectors and pyrodetectors, have generally consisted of a thin pyroelectric single crystal with one electrode each on the faces perpendicular to the polar axis. See, for example, J. Appl. Phys. Vol. 44, No. 2, pages 929 -931 (Feb. 1973). A problem in the preparation of such pyroelectric radiation detectors generally consists in the fact that the single crystal must be accurately cut so that the planes of the electrode surfaces are perpendicular to the ferroelectric axis of the crystal. Moreover, the preferred material for pyroelectric detectors has been triglycine sulfate (TGS) which in the form of a single crystal is very brittle and must be handled with extreme care during preparation and use of the detector.
In response to these problems U.S. Pat. No. 3,511,991 (German Offenlegungsschrift No. 19 05 197) and British Patent No. 1,377,625) disclosed the preparation of pyroelectric detector elements by applying pyroelectric microcrystals, e.g. TGS or lithium sulfate monohydrate, to a substrate by means of a binder, and orienting the ferroelectric axes of the microcrystals by a one-time application of a polarizing d.c. voltage.
The pyroelectric detector disclosed in U.S. Pat. No. 3,511,991 has been found to have a number of drawbacks. For example, triglycine salts have a Curie temperature of less than 50.degree. C. and are unsuitable where temperatures exceed about 50.degree. C. since depolarization will occur and the detector will no longer deliver a signal. Accordingly, since pyroelectric detectors are generally subjected to temperatures of about 150 to 200.degree. C., e.g. in curing binding agents, soldering, drying etc., triglycine salts are not feasible. The prior art '991 patent further discloses that the maximum content of thermoplastic binder is 25% by weight of the triglycine salt (microcrystals). However, it has been found that a higher proportion of binder is desirable since it enables the dielectric constant of the pyroelectric material to be reduced without causing any significant change in the pyroelectric coefficient. Another drawback is that a thermoplastic binder will become soft upon poling at high temperatures. Finally, the lithium sulfate monohydrate disclosed as suitable pyroelectric material in U.S. Pat. No. 3,511,991 is exceedingly hygroscopic and this property results in pyrodetectors having unacceptably low d.c. resistance and a signal drop.
British Patent No. 1,377,625 discloses a pyrodetector which consists of a pyroelectric ceramic distributed in an electrically insulating binder. However the quality coefficient of this pyrodetector is insufficient. The pyroelectric quality coefficient G is defined as ##EQU1## wherein p is the pyroelectric coefficient in .differential.C/m.sup.2.K, DK the dielectric constant, D the density in g/cm.sup.3 and c the heat capacity in J/g.K).
Accordingly, the object of the present invention is to provide a pyroelectric detector which avoids the drawbacks of the pyroelectric detectors discussed above. More specifically, the object of the invention is to provide a pyroelectric detector which has an elevated Curie temperature e.g. about 610.degree. C.; which undergoes no depolarization during the preparation and operation of the detector; in which the pyroelectric coefficient p and the dielectric constant DK are practically temperature-independent in the range of -30.degree. C. to +80.degree. C.; which has a considerably improved quality coefficient compared to the pyrodetectors known in the art; which has a high d.c. resistance; and which uses a nonhygroscopic material, so that aging through water uptake is prevented.
These objectives are met by the pyroelectric detector of the type described and illustrated hereinbelow and the process disclosed for making the same.